Measuring apparatus



July 30, 1946. WILLS 2,404,894

MEASURING APPARATUS Original Filed Nov. 15, 1938 FIG. 1.

ZNVENTOR. WALTER P. WILLS Patented July 30, 1946 MEASURING APPARATUSWalter P. Wills, Philadelphia, Pa., assignor to The Brown InstrumentCompany, Philadelphia, Pa., a corporation of Pennsylvania Originalapplication November 15, 1938, Serial No.

240,594. Divided and this application September 25, 1942, Serial No.459,638

12 Claims. 1

A prior application Serial No. 240,594, filed November 15, 1938, byThomas R. Harrison and myself as joint inventors and issued into Patent2,300,742 on November 3, 1942, discloses certain improvements in methodsof and apparatus for preventing hunting in automatic recording andcontrolling systems. Said prior application discloses certainimprovements in methods of and apparatus for the measurement of minuteelectrical currents or potentials and their utilization for control andanalogous purposes which were not the joint invention of the applicantswho made said prior application and are my sole invention and aredisclosed and claimed by me in the present application, which under thecircumstances is to be regarded as a division of said prior application.

A general object of the invention is to provide an improved method ofmeasuring and/or recording potential or current variations of minutemagnitude.

A more specific object of the invention is to provide a. method ofemploying current or potential variations of' minute magnitude tocontrol the operation of electro-mechanical apparatus.

A still more specific object of the invention is to provide an improvedmethod of and apparatus for eliminating the efiects of spuriouselectrical eifects upon the operation of apparatus employed formeasuring the magnitude and changes in magnitude of minute electricalcurrents or potentials.

It is a particular object of the invention to provide a method of andapparatus for preventing the introduction of extraneous alternatingcurrents into the measuring circuit of apparatus utilized for makingmeasurements of minute unidirectional currents or potentials in lowresistance circuits.

A serious problem in the measurement of minute unidirectional potentialor current variations in low resistance circuits, particularly inpotentiometric measuring circuits wherein the source of minuteunidirectional potential or current is derived from a thermocouple, isthe chiliculty of electrically amplifying such potential or currentvariations. The direct amplification of minute unidirectional or currentvariations in low resistance circuits by means of conventionalelectronic amplifying circuits is difficult because changes in thespacing and relative positions of the electrodes of the amplifying tubesproduce effects which are similar to and are of the same order ofmagnitude as the. changes inthe. minute.

unidirectional potential or current under measurement.

When the source of minute unidirectional potential or current undermeasurement is derived from a thermocouple which is located in afurnace, the temperature of which it is desired to ascertain, additionalproblems in the measurement of the minute unidirectional potential orcurrent are encountered because of the introduction of extraneousalternating currents into the measuring circuit from leakage paths inthe furnace between the thermocouple and ground. By way of example, whenthe furnace whose temperature under measurement is an electric furnacewhich has heating resistance elements disposed in its walls, alternatingcurrents frequently flow from the furnace heating elements to groundthrough leakage paths in the furnace walls. The thermocouple extendsthrough the furnace walls and even though it is provided with aprotecting tube some of the alternating or fluctuating currents whichfiow in the furnace Wall leakage paths tend to flow through thethermocouple and the measuring circuit to ground. The thermocouple andmeasuring circuit provide a shunt path for such leakage currents toground since a portion of the apparatus utilized for measuring thepotential or current variations of the thermocouple, generally theelectronic amplifier is grounded. It is noted that fluctuating currentswill also tend to be established in the thermocouple and measuringcircuit due to electrolytic action between the thermocouple and ground.The alternating or fluctuating currents introduced into the thermocoupleand measuring circuit produce inaccuracies in the measurements obtainedand render the operation of the apparatus unstable. More specifically,when a potentiometric measuring arrangement of the self balancing typeis employed in conjunction with the thermocouple and electronicamplifier, such extraneous alternating currents in the thermocouple andpotentiometer circuit introduce false balance points intothepotentiometer and also cause erratic operation of the potentiometricrebalancing motive structure.

It is accordingly a primary object of the pres sent invention to providean improved method of and apparatus for measuring minute unidirectionalpotential or current variations produced by a thermocouple in which thedifiiculties pointed out above have been wholly eliminated or materiallyminimized.

This advantageous result is obtainedin accordance with the presentinvention by isolating the thermocouple from the potentiometricmeasuring network and the associated electronic amplifying apparatus. Tothat end the thermocouple is periodically connected to the terminals ofa condenser of suitable value which is insulated from ground, and duringalternate intervals the condenser is disconnected from the thermocoupleand is connected to the input terminals of the potentiometric measuringnetwork and the associated electronic amplifier. By this means anyextraneous alternating or fluctuating currents which may exist in anyleakage paths from the thermocouple to ground are prevented from flowingthrough the potentiometric measuring network and associated electronicamplifier, and therefore, are rendered incapable of affecting theoperation of the measuring apparatus. Since the condenser is insulatedfrom ground, no leakage currents tend to flow to the condenser from thefurnace leakage paths,

The various features of novelty which characterize my invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,however, its advantages and specific objects obtained with its use,reference should be had to the accompanying drawing and descriptivematter in which I have illustrated and described a preferred embodimentof the invention.

Of the drawing:

Fig. l is a diagrammatic representation of the use of the invention in apotentiometric recording and controlling system;

Figs. 2 and 3 illustrate in detail a form of interrupter that may beemployed in the arrangement of Fig. 1; and

Fig. 4 illustrates schematically a form of electronic amplifier that maybe employed in the Fig. 1 arrangement.

Referring more particularly to Fig. 1 of the drawing, there isillustrated in schematic form an arrangement including an electronicdevice l shown in detail in Fig. 4 for producing effects in accordancewith the extent of unbalance of a potentiometric measuring network whichcontrols the electronic device and is unbalanced in ac-- cordance withthe variations in a quantity to be measured and in which because of thesmall magnitude of the unbalance electromotive forces, it is notpractical nor desirable to have the said effects produced directly bythe potentiometric measuring network.

More specifically, an arrangement is illustrated in Fig. l for recordingand controlling the temperature of a furnace 2 in the interior of whicha thermocouple 3 is arranged in heat transfer relation therewith and isresponsive to slight changes in furnace temperature. The thermocouplewhich may be located at a distance from the remainder of the measuringapparatus has its terminals connected by a pair of conductors l and 3 9adapted to ride on a screw threaded rod Iii which is rotated in onedirection or the other under control of the thermocouple 3. A suitablereversible electrical motor H is provided and is coupled in anyconvenient manner to the screw threaded rod ii! to rotate the latter atthe desired speed and in the desired direction and thereby to move thcontact 8 along the slidewire resistance l to rebalance thepotentiometer 6 when the latter is unbalanced.

The terminals of the thermocouple 3 are connected by the conductors 3and to the input to the terminals of a null point potentiometricmeasuring network 5. The potentiometric measuring network 5 includes aslidewire resistance '7 and an associated contact 8 which is capable ofbeing moved along the length of the slidewire and may be of any suitabletype for example such as the Brown potentiometric measuring networkdisclosed in Patent 1,898,124 issued to Thomas R, Harrison on February21, 1933.

The movable contact 8 of the potentiometer is attached to a suitablecarrier which for example may be in the form of an internally threadednut terminals [2 and 13 respectively of a double contact interrupter Mwhich is illustrated in Figs. 2 and 3. When in one position, theinterrupter it operates to apply the potential developed by thethermocouple 3 to the terminals of a condenser l5 which is suitablyinsulated from ground. To this end, the interrupter I4 is provided witha movable pair of contacts l6 and ll which are insulated from each otherand from contacts l2 and i3 and are adapted to be moved into engagementwith the contacts 52 and 113, respectively, when the interrupter i4 isin said one position. When the interrupter is in its other position themovable contacts l6 and H are in engagement with a pair of insulatedcontacts l8 and !9, respectively, and in this position of theinterrupter the potential which was impressed on the condenser l5 by thethermocouple 3 is connected in opposition to the potentiometricpotential between the left end terminal of the slidewire resistance 1and the contact 3. It should be noted that in the last mentionedposition of the interrupter M the thermocouple 3 is totally disconnectedfrom the remainder of the measuring apparatus, and therefore, iseiiectively isolated from the remainder of the apparatus. In this mannerthe introduction of any extraneous fluctuating or alternating cur rentsin the thermocouple circuit due to leakage paths which may beestablished between the thermocouple and ground through the walls of thefurnace due to electrolytic action between the thermocouple and groundis eifectively prevented from affecting the operation of the measuringapparatus. In addition, since the condenser !5 is insulated from ground,n0 extraneous fluctuating or alternating currents will flow to thecondenser from leakage paths between the thermocouple and ground.

The form of interrupter l4 shown in Figs. 2 and 3 includes acontinuously rotating shaft 28 which may be driven by any suitable formof motor such as a synchronous electric motor 2! which receivesenergizing current from supply conductors not shown and on which shaftare insulatingly mounted two pairs of slip rings 22, 23 and 24, 25 and afour segment commutator 26 each segment being of the same arcuate lengthand all insulated from each other. Brushes l6 and I! are provided forthe commutator 2'5 and corresponding brushes l2, l3, l8 and 19 areprovided for the slip rings 22, 23, 24 and '25, respectively. Brushes l6and H are connected to opposite terminals of the condenser 15 while thebrushes [2 and i3 are connected to conductors 4 and 5, respectively, andthe brushes l8 and is are connected to one end terminal of the slidewireresistance l and to contact 8, respectively. One opposite pair ofsegments On the commutator 25 are connected to the slip rings 22 and 23and the other opposite pair of segments are connected to the slip rings24 and 25. Thus, during each cycle of rotation of the shaft 20, one pairof segments on the commutator will be in engagement with the brushes [6and I1 for one half cycle and during'that time will close the circuitfrom the thermocouple leads 4 and to the condenser l5. During theremaining half cycle the other pair of commutatorsegments will be inengagement with the said brushes to connect the potentiometer slidewireresistance I in circuit with the con-- denser 15. The speed of the motor21 which drives the shaft 29- is so adjusted that the time for one cycleof rotation of the shaft is identical with the time of one cycle of thealternating current supplied by the conductors L and L in Fig. 1. Thereason for so relating the speed of rotation of the motor 2! and thetime of one cycle of the alternatin-gcurrent supplied by lines L and Lis made apparent hereinafter.

The periodic connection of the potential produced on the condenser H3 inopposition to the potential tapped on the potentiometer slidewireresistance 1 produces a pulsating drop across a resistance 2! connectedin the potentiometer circuit, which potential drop is either in phasewith the voltage of the supply lines L and L or is displaced 180 inphase therefrom. This pulsating potential drop i impressed on the inputterminals of the amplifier I wherein it is amplified and the amplifiedquantity is applied to the terminals of one winding '28 or 29 01" thereversible electrical motor H which as illustrated in detail in Fig. 4also includes a winding 39 which is connected to the alternating currentsupply lines L and L through a suitable condenser 3|,

The reversible electrical motor H is of the induction variety andincludes a squirrel cage. rotor and two pairs of oppositely disposedfield poles on which the windings 28, 29 and 39 are wound. Winding 28 iswound on one field pole of one of said pairs and winding 29 is wound onthe other field pole of that pair. Winding 3B is wound on the other pairof field poles and due to the action of condenser 3| the current whichflows through the windin 39 will lead the line voltage by approximately90. The current supplied the winding 28 by the amplifier l is in phasewith the supply line voltage and establishes a field in the rotor whichi displaced 90 in the forward direction with respect to that establishedtherein by the winding 30. Similarly, the current supplied winding 28 byamplifier l is in phase with the supply lin voltage, but since thewinding 29 is wound on an opposite field pole from that on which thewinding 28 is wound, winding 29 establishes a field in the rotor whichlags by 90 that established by winding 3|]. Reaction between the fieldset up by winding 28 or 29 with that set up by winding 30 establishes arotating field in the rotor which rotates in one direction or the otherdependent upon whether winding 28 or 29 is energized and thus on thedirection of potentiometric unbalance. The motor rotor is connectedthrough suitable gearing or couplings to the screw threaded shaft [9 sothat the contact 8 is.ad justed along the slidewire resistance 1 inaccordance with the direction of rotation of the rotor. The directionand duration of rotation of the rotor is controlled by the direction andextent of unbalance of the potentiometer so that on motor rotation, thecontact 8 is adjusted in the proper direction to reduce thepotentiometer unbalance.

If desired, a pen may be mounted on the carriage 9 which carrie thepotentiometer contact 8 and arranged in cooperative relation with arecorder chart 32 to thereby provide a continuous record of thetemperature of the furnace in which the thermocouple 3 is inserted. Thechart 32' may 6 be a strip chart as shown and is adapted to be driven inany convenient manner as for example by'a undirectional motor 33 throughsuitable gearing (not shown), so that a record of the temperature towhich the thermocouple v3 is subjected will be recorded as a continuousline on the chart.

The electronic amplifier i as noted hereinbefore is illustrated indetail in Fig. 4 and as shown includes an electronic valve 34 which ispreferably a heater type high mu triode having an anode, cathode and acontrol electrode and having its input circuit connected by conductors35 and 36 to the terminals of the resistance 2?. Anode voltage issupplied the valve 34 from the terminals of a suitable filter 3'! whichis connected in circuit between the valve 34 and a. rectifier 38. Therectifier 38 is a conventional full wave rectifier employing a rectifiervalve 39 including two heater type diodes in one envelope. Energizingcurrent is supplied the heater filaments of the diodes from the lowvoltage secondary winding 40 of a transformer it which also includes aline voltage primary winding 42, a high voltage secondary winding 43 andan additional low voltage secondary Windin 44. The line voltage primarywinding :22 is connected to the alternating current supply lines L and LThe anode of the one diode of valve 35 is connected to one terminal ofthe winding- 43 and the anode of the second diode is connected to theother terminal of the winding 43. The cathodes of the diodes areconnected together and through a resistance 45 to the positive terminalof the filter 31 and the negative terminal of the latter is connected toa center-tap on the winding 43 and to a center-tap on the winding 4%).The negative terminal of the filter is desirably connected to groundpotential as shown.

The filter 3! includes a condenser 46 which shunt its positive andnegative terminals and has its positive terminal connected to the anodeof valve 34 through resistances 41, 48 and 49, and has its negativeterminal connected directly to the cathode of said valve. Asillustrated, the point of engagement of resistances 4i and 48 isconnected by a condenser 50 to the negative terminal of the filter andthe point of engagement of resistances 48 and 49 is connected by acondenser 5| thereto.

Energizing current is supplied the heater filament of valve 34 from thelow voltage transformer secondary winding 44 which also suppliesenergizing current to the heater filaments of a twin type electronicvalve 52. The flow of current through valve 34 is normally maintained ata mean value ince the resistance 21 is connected directly across theinput circuit thereof, but when a pulsating potential appears across theterminals of resistance 21, the conductivity of valve 34 is alternatelyincreased and decreased resulting in a pulsating potential dropappearing across the resistance 49 in the output circuit of the valve34.

The output circuit of valve 34 is resistance capacity coupled to theinput circuit of valve 52 through a condenser 53 and a resistance 54connected across the input circuit of valve 52. Valve 52 is a heatertype valve including two triodes in one envelope. Each triode includesanode, cathode and control electrode elements. For convenience, thetriode having the resistance 54 connected across it input circuit willbe referred to as the triode A and the second triode will be referred toas the triode B.

Anode voltage is supplied the triodes A and B from the terminals of thefilter 31, and as shown, the anode of triode A is connected through aresistance 55 to the point of engagement Of resistances 41 and 48, andthe anode of trio-de B is connected through the primary winding 56 of atransformer I to the positive terminal of the filter. The cathodes oftriodes A and B are connected together and to the negative terminal ofthe filter.

The output circuit of triode A is resistance capacity coupled by acondenser 58 and a resistance 59 to the input circuit of the triode Band the output circuit of the latter is coupled by transformer 51 to theinput circuit of a pair of electronic valves 59 and 65 which areconnected in push-pull. The transformer 51 includes a center tappedsecondary winding 5|, the terminals of which are connected to arespective control electrode of the valves 59 and 60 and the center tapof which is connected through a biasing resistance 62 to the cathodes ofthe valves, which, as shown, are connected together. As illustrated, acondenser 63 may desirably be connected across the terminals of thetransformer secondary winding 5| for tuning the latter to the frequencyit is desired to amplify. Valves 59 and 60 are heater type tetrodes andinclude anode, cathode, heater filament, control electrode, and screengrid elements. The heater filaments of the valves 59 and 59 areenergized from the Winding 44.

Anode voltage may be supplied the valves 59 and 60 directly from thesupply conductors L and L as shown, or may be supplied thereto from asuitable transformer energized by the supply line current, if desired.Winding 28 of motor II is connected in the anode circuit of valve 59 andwinding 29 of the motor is connected in the anode circuit of valve 60.

In operation, when a pulsating potential drop is produced acrossresistance ZI as a result of potentiometer unbalance, the resultingamplified pulsating current flows through the transformer primarywinding 55 will cause the induction of an alternating voltage across theterminals of the transformer secondary winding 6| Which voltage isimpressed on the input circuit of valves 59 and 68. The alternatingvoltage which appears across the terminals of the transformer secondarywinding 6| swings the potentials of the control electrodes of the valves59 and 50 in opposite phase at a frequency corresponding to the supplyline frequency and thereby renders one valve or the other non-conductivedepending upon the phase of the voltage of the transfermer secondarywinding 6| with respect to the supply line voltage. The resultingdeenergization of one motor winding 28 Or 29 and the increasedenergization of the other operates to produce rotation of the motor inone direction or the other depending upon the phase of the pulsatingpotential drop produced across resistance ZI and thereby the directionof potentiometer unbalance. As illustrated, a condenser 64 may bedesirably connected between the anodes of valves 59 and 69 to increasethe available torque of the motor I I.

In order that the speed of the motor I I may be as great as possiblewithout overshooting of the new balance point of the potentiometricnetwork 6 and consequent hunting taking place, means have been providedto insure that the motor speed is reduced to zero as the balance pointis reached. This end is obtained by the arrangement including thecondenser I5 and a resistance 65 which is inserted in the conductor 5leading from the interrupter contact I3 to the thermocouple 3, as shown.

The operation or this arrangement including condenser I5 and resistance65 in eliminating overshooting and consequent hunting of thepotentiometric system is described in detail in the prior application,Serial No. 240,594 filed by Thomas R. Harrison and myself and referredto hereinbefore and therefore will only be briefly described herein.

With the resistance 55 and condenser I5 a ranged as shown, it will beapparent that when the interrupter contacts I5 and I? are in engagementwith the contacts I? and I3, the thermocouple 3 operates to charge thecondenser I5 through the resistance 55 and the electromotive force thusdeveloped between the condenser terminals is thereafter compared withthe potentiometer electromotive force at the then position of thecontact 8 along the slidewire resistance 1 when the interrupter contactsI6 and H are moved into engagement with the contacts I8 and I9,respectively.

With the temperature of the furnace to which the thermocouple isresponsive at a predetermined value, the condenser I5 will tend to becharged through the resistance 65 until the condenser electromotiveforce is equal to that of the thermocouple. The contact 8 would then bein a position along the slidewire resistance I such that theelectromotive force tapped off the slidewire I is exactly equal andopposite to the condenser electromotive force. For convenience, when theslidewire electromotive force is referred to hereinafter, that portiontapped off resistance I and opposed to the condenser electromotive forceis the electromotive force intended. On a change in the temperature ofthe furnace, for example on an increase in temperature, the thermalelectromotive force will increase the electromotive force developedacross the condenser terminals. The fiow of charging current to thecondenser through resistance 65 will produce a potential drop across thelatter, and as a result, the electromotive force developed across thecondenser terminals will not assume the final value of the thermalelectromotive force until the current through resistance 65 is reducedto zero or in other words until the system is again balanced. Thus,until the slidewire electromotive force is adjusted to the new value ofthe thermocouple electromotive force, the condenser electromotive forcewill tend to assume a value intermediate the thermocouple and slidewireelectromotive forces. The flow of current through resistance 2! onunbalance of the condenser and potentiometer electromotive forces willoperate substantially immediately to produce energization of motor IIfor rotation to effect adjustment of the contact 8 in the properdirection to reduce the unbalance between the condenser andpotentiometer electromotive forces.

It is noted that there is no delaying means in the circuit through whichthe condenser and slidewire electromotive forces are opposed so that theamplifier I responds substantially immediately to unbalance in saidelectromotive forces to energize the motor II for rotation in onedirection or the other to adjust the slidewire electromotive force asrequired to reduce the unbalance and reduce the motor energization tozero at the instant the balance between said electromotive force isrestored. Due to the inertia of the motor, however, the speed of thelatter will not fall off as quickly as the energization thereof andconsequently, the slidewise electromotive force will tend to overshootthe value of the condenser electromotive force. As a result, thepotentiometric network will be momentarily unbalanced in the oppositedirection, which unbalance will produce an effect energizing the motorfor rotation in the reverse direction to thereby quickly decelerate thelatter. Inasmuch as the condenser electromotive force differs from thethermocouple electromotive force by an amount equal to the potentialdrop produced across resistance 65 by the fiow of current therethrough,the contact 8 will not have reached the position along slidewireresistance 1 correspond ing to the new value of thermocoupleelectromotive force at the instant when the condenser and slidewireelectromotive forces were exactly ba anced. After the condenser andslidewire electromotive forces are balanced, the condenser will notassumethe thermocouple electromotive force until after the lapse of apredetermined interval required to charge the condenser to thethermocouple potential and by making this interval of the properduration, the motor will be decelerated and ease the contact 8-gradually into its true balanced position without overshooting. Theproper adjustment of the duration of the lag between the condenser andthermocouple electromotive forces may be readily effected by properlyproportioning resistance 65 and condenser H5 in relation to theeffective resistance of the circuit including the slidewire resistance 1and resistance 21. When the circuit components are properlyproportioned, the motor may be extremely fast in its rebalancing effectand is capa ble of moving the contact 8 completely along the length ofthe slidewire resistance 1, a distance of approximately 12" in somecases, in a fraction of a second without overshooting and consequenthunting taking place.

It will be apparent that the motor H may be employed to operate acontrol valve or rheostat for controlling the supply of heating agent tothe furnace 2 to the temperature of which the thermocouple 3 isresponsive, or another motor desirably operated together with the motorI I may be so employed. For example, as shown in Fig. I, the furnace 2may be heated by a resistance 66 which is connected to electric supplyconductors L and L through a rheostat 61 the adjustment of which may beeffected by a motor 68. The motor 68 may be exactly like motorll and isconnected in parallel therewith. The mechanical connection of therheostat 67 to the motor 68 is such as to increase and decrease thesupply of electric current to the resistance 66 as the temperature towhich the thermocouple 3 is responsive drops below and rises above apredetermined level.

While in accordance with the provision of the statutes, I haveillustrated and described the best forms of embodiment of my inventionnow known to me, it will be apparent to those skilled in the art thatchanges may be made in the forms of the apparatus disclosed withoutdeparting from the spirit of my invention, as set forth in the appendedclaims and that in some case certain features of my invention may beused to advantage without a corresponding use of other features.

Havingnow described my invention, what I claim as new and desire tosecure by Letters Patent is:

1. The method which comprises producing an E. M. F. to be measured,periodically impressing said E. M. F. on an electrical energy storingdevice to charge the latter, opposing the E. M. F. on said device to aknown E. M. F. during each interval when said first mentioned E. M. F.is not impressed on said device whereby the resultant of said opposed E.M. F.s creates acurrent of a regular frequency which can be readilyamplified, amplifying said current, and applying said amplified currentto effect a balance between said opposited E. M. F.s.

2. The method which comprse producing an M. F. to be measured,periodically impressing said E. M. F. on an electrical capacitivereactance to charge the latter, opposing the E. M. F. on said reactanceto a known E. M. F. during each interval when said first mentioned E. M.F. is not impressed on said device whereby the resultant of said opposedE. M. F.s creates a current of regular frequency which can be readilyamplified, amplifying said current at said frequency,

and pp ying said amplified currents to effect a balance between saidopposed E. M. F.s.

3. In measuring apparatus, a circuit including potentiometer resistance,a standard source of i for said resistance, an electrical energy..tcring device, means to periodically connect a scurc of E. M. F. to bemeasured to said device to charge the latter and to connect said devicein said circuit to oppose the E. M. F. on said device to said standardE. M. F. during each interval when said E. M. F. to be measured is notconnected to said device to thereby create a pulsating current flow ofregular frequency in said circuit, means to amplify said pulsatingcurrent, and means energized by said amplified quantity to eifect abalance between said opposed E. M. F.s.

4. In measuring apparatus, a circuit including a potentiometerresistance, a standard source of E. M. F. for said resistance, anelectrical capacitive reactance, means to periodically connect a sourceof E. M. F. to be measured to said reactance to charge the latter and toconnect said reactance in said circuit to oppose the E. M. F. on saidreactance to said standard E. M. F. during each interval when said E. M.F. to be measured is not connected to said reactance to thereby create apulsating current flow of regular fre quency in said circuit, means toamplify said pulsating current, and means energized by said amplifiedquantity to adjust said potentiometer resistance to effect a. balancebetween said opposed E. M. F.s.

5. In measuring apparatus, a circuit including a potentiometerresistance, a, standard source of E. M. F. for said resistance, anelectrical capacitive reactance, means to periodically connect a sourceof E. M. F. to be measured to said reactance to charge the latter and toconnect said reactance in said circuit to oppose the E. M. F. on saidreactance to said standard E. M. F. during each interval when said E. M.F. to be measured is not connected to said reactance to thereby create apulsating current flow of regular frequency in said circuit, means toamplify said pulsating current at said frequency, a source ofalternating current of said frequency, a two phase rotating field moto;connected to said potentiometer resistance for adjustment of the latterand having one phase energized from said source, and means to apply saidamplified quantity to the other phase of said motor to control theoperation of the latter and thereby the adjustment of said potentiometerresistance as required to effect a balance between said opposed E. M.F.s.

6. In measuring apparatus, a circuit including a potentiometerresistance, a standard source of E. M. F. for said resistance, anelectrical capacitive reactance, means to periodically connect a sourceof E. M. F. to be measured to said reactance to charge the latter and toconnect said reactance in said circuit to oppose the E. M. F. on saidreactance to said standard E. M. F. during each interval when said E. M.F. to be measured 18 not connected to said reactance to thereby create apulsating current flow of regular frequency in said circuit, means toamplify said pulsating current, and means to utilize said amplifiedquantity to effect a balance between said standard E. M. F. and the E.M. F. on said reactance.

7. In measuring apparatus, a circuit including a standard source of E.M. F., an electrical energy storing device, means to periodicallyconnect a source of E. M. F. to be measured to said device to charge thelatter and to connect said device in said circuit to oppose the E. M. F.on said device to said standard E. M. F. during each interval when saidE. M. F. to be measured is not connected to said device to therebycreate a pulsating current flow of regular frequency in said circuit,means to amplify said pulsating current, and means to utilize saidamplified quantity to effect a balance between said standard E. M. F.and the E. M. F. on said device.

8. Measuring apparatus including means for producing a variable E. M. F.to be measured, means for producing a standard E. M. F., a device havinginertia for varying said standard E. M. F., a circuit in which saidstandard E. M. F. is permanentlyconnected, an electrical capacitivereactance, a resistance, means to periodically connect said firstmentioned E. M. F., said resistance and said reactance in series tocharge the latter and to connect said reactance in said circuit tooppose the E. M. F. on said reactance to said standard E. M. F. duringeach interval when the E. M. F. to be measured is not connected to saidreactance to thereby create a pulsating current flow of regularfrequency in said circuit, means to amplify said pulsating current, andmeans to apply the amplified quantity to said device to control theoperation of the latter, said resistance and reactance cooperating toautomatically control the E. M. F. on said reactance in a predeterminedmanner with respect to time on a change in said first mentioned E. M. F.so as to cause a balance of the opposed E. M. F.s in said circuit whenthe difference between said first mentioned E. M. F. and said standardE. M. F. is equal to the E. M. F. produced by said device due to itsinertia following deenergization of the latter.

9. Measuring apparatus including means for producing a variable E. M. F.to be measured, means for producing a standard E. M. F., a device forvarying said standard E. M. F., a motor arranged to adjust said devicewhen energized and having the inertia characteristic which producesfurther adjustment of said device following deenergization, a circuit inwhich said standard E. M. F. is permanently connected, an electricalcapacitive reactance, a resistance, means to periodically connect saidfirst mentioned E. M. F., said resistance and said reactance in seriesto charge the latter and to connect said reactance. in said circuit tooppose the E. M. F. on said reactance to said standard E. M. F. duringeach interval when the E. M. F. to be measured is not connected to saidreactance to thereby create a pulsating current fiow of regularfrequency in said circuit, means to amplify said pulsating current, andmeans to apply the amplified quantity to said motor to control theoperation of the latter, said resistance and reactance cooperating toautomatically control the E. M. F. on said reactance in a predeterminedmanner with respect to time on a change in said first mentioned E. M. F.so as to cause a balance of the opposed E. M. F.s in said circuit whenthe difference between said first mentioned E. M. F. and said standardE. M. F. is equal to the E. M. F. produced by said device under controlof said motor following deenergization of the latter.

10. In measuring apparatus, a circuit including a potentiometerresistance, a standard source of E. M. F. for said resistance, anelectrical energy storing device, a double pole-double throw switch toperiodically connect a source of E. M. F. to be measured to said deviceto charge the latter and to totally disconnect said device from said E.M. F. to be measured and to connect said device in said circuit tooppose the E. M. F. on said device to said standard E. M. F. during eachinterval when said E. M. F. to be measured is not connected to saiddevice to thereby create a pulsating current flow of regular frequencyin said circuit, means to amplify said pulsating cur-- rent, and meansenergized by said amplified quantity to effect a balance between saidopposed E. M. F.s.

11. In a measuring apparatus, a circuit including a potentiometerresistance, a standard source of E. M. F. for said resistance, anelectrical capacitive reactance, a double pole-double throw switch toperiodically connect a source of E. M. F. to be measured to saidreactance to charge the latter and to totally disconnect said reactancefrom said E. M. F. to be measured and to connect said reactance in saidcircuit to oppose the E. M. F. on said reactance to said standard E. M.F. during each interval when said E. M. F. to be measured is notconnected to said reactance to thereby create a pulsating current fiowof regular frequency in said circuit, means to amplify said pulsatingcurrent at said frequency, a source of alternating current of saidfrequency, a two phase rotating field motor connected to saidpotentiometer resistance for adjustment of the latter and having onephase energized from said source, and means to apply said amplifiedquantity to the other phase of said motor to control the operation ofthe latter and thereby the adjustment of said potentiometer resistanceas required to effect a balance between said opposed E. M. Fs.

12. Measuring apparatus including means for producing a variable E. M.F. to be measured. means'for producing a standard E. M. F., a device forvarying said standard E. M. F., a motor arranged to adjust said devicewhen energized and having the inertia characteristic which producesfurther adjustment of said device following dcenergization, a circuit inwhich said standard E. M. F. is permanently connected, an electricalcapacitive reactance, a resistance, a double polethrow switch toperiodically connect said first mentioned E. M. F., said resistance andsaid re actance in series to charge the latter and to totally disconnectsaid reactance from said first mentioned E. M. F. and to connect saidreac-- tance in said circuit to oppose the E .M. F. on

said reactance to said standard E. M. F. during quency in said circuit,means to amplify said pul- 1; eating current, and means to apply theampli- 13 fied quantity to said motor to control the operation of thelatter, said resistance and reactance cooperating to automaticallycontrol the E. M. F. on said reactance in a predetermined manner theopposed E. M. F.s in said circuit when the difference between said firstmentioned E. M. F. and said standard E. M. F. is equal to the E. M. F.produced by said device under control of said with respect to time on a.change in said first 5 motor following deenergization of the latter.

mentioned E. M. F. so as to cause a balance of WALTER P. WILLS.

